1. Technical Field of the Invention
The present invention relates generally to an apparatus designed to control a dynamic system such as a continuously variable transmission for automotive vehicles under the sliding mode and feedforward control.
2. Background of Related Art
In recent years, belt-driven continuously variable transmissions are used in automotive vehicles. This type of transmissions are usually controlled to achieve a gear ratio corresponding to an operating condition of the vehicle by changing parameters such as the position of a driving pulley and the primary speed (i.e., the speed of the driving pulley).
For adjustment of the gear ratio, the belt-driven continuously variable transmission includes a secondary hydraulic system which is responsive to an input torque to develop a secondary hydraulic pressure for adjustment of belt-holding force of a driven pulley and a gear ratio control system which is responsive to the input torque and the secondary hydraulic pressure to develop a primary hydraulic pressure required for adjustment of the driving pulley to change the gear ratio. To establish a desired gear ratio, the gear ratio control system calculates a target pulley position and a target primary speed to determine differences or shifts of actual pulley position and primary speed from the targets and calculates a target primary hydraulic pressure under feedback control based on the determined shifts.
In the feedback control, adjustment of a control gain or correction of a target controlled parameter is performed according to several operating modes of the vehicle such as starting, steady running, kickdown, manual gear shifting, and braking to avoid the hunting, thereby optimizing the response rate and convergence of shifting control.
Japanese Patent No. 2505420, as an example of the control gain adjustment, teaches increasing the control gain at sudden deceleration or braking over that during normal traveling to speed up the upshift operation.
Japanese Patent First Publication No. 7-167234, as an example of the correction of the target controlled parameter, teaches limiting the speed at which the target primary speed is reduced for avoiding the transmission hunting due to undershooting of an actual primary speed resulting from excessive integral control activity by a PID controller when the target primary speed is decreased suddenly, for example, by rapid closing of a throttle valve of the vehicle.
The above conventional techniques, however, encounter drawbacks, as discussed below, due to the nature of PID control.
(1) Control software is complex because of control gain adjustment or target parameter correction performed in each operating mode of the vehicle.
(2) It is necessary to adjust a target control gain or the amount of correction of a target controlled parameter in each operating mode of the vehicle, and the development of software consumes considerable time.
As feedback control for alleviating the disadvantages of the PID control, the sliding mode control is known which has high robustness and hardly accounts for the above problems (1) and (2).
For example, Japanese Patent First Publication Nos. 8-249076, 61-271509, and 61-271510 and U.S. Pat. No. 5,216,342 disclose the so-called sliding mode control which subjects a servo mechanism to feedback control through a sliding mode controller based on a target value determined by a feedforward operation.
Usually, the sliding mode control involves the hunting, which would be not objectionable in the field of dynamic systems such as robot manipulators in which operating requirements are limited, but will be objectionable in control of, for example, a continuously variable transmission for automotive vehicles, thereby resulting in an uncomfortable ride.